The present invention relates to a video storage type communication system and more particularly to a video storage type communication device which is capable of successively decoding received coded video data by video decoding means, encoding the decoded video frame/field (hereinafter simply referred to as frame) only in intraframe mode by coding means and stored in a center storage device, thereby realizing the possibility of generating pictures specially reproducible, for example, with rapid feeding.
Recently, with the spread and progress of digital transmission lines and the advance of image processing technology based upon the development of high speed digital signal processing methods and related LSI technology, there has been an increasing demand for development of effective method of using video information services using high-speed digital communication networks, e.g., ISDN (Integrated Services Digital Networks). Video telecommunication services and a video conferencing services are well known as currently available video communication services. The most newly emerging services are video information database services which allow each user by using his audiovisual terminal set (referred to as a terminal) to access a database storage (recording device) of a host center and to obtain desired video information therefrom. One of these systems is a storage type communication device for use in a host center, which is intended to control storing video information in a center storage and reproducing the video information at each terminal from the host center.
A conventional storage type communication system for providing a video storage-and-delivery service has a video storage type communication device, a video storage portion, a communication control portion, a receiving portion, a transmitting portion and etc., and a video data coded by any one of the terminals is transmitted over the communication network to the receiving portion. The receiving portion divides a received coded video-data into frames and transmits the frames to the video storage portion, and the video storage portion stores the received coded video-data (frames) therein according to an instruction given by the communication control portion. The video storage type communication device is intended to be connected with an audiovisual terminal (hereinafter abbreviated to AV terminal) which conforms the recommendation ITU-T (International Telecommunications Union) and meets the recommendation H.261 on a video information coding system.
In the video storage and communication services, it is desired to prepare functions of video reproduction with rapid forward or rapid reverse feeding. However, video data coded according to the recommendation H.261 is usually coded by interframe coding method and therefore consists of differential information. If a first frame data is interframely coded, each receiving terminal can not restore the frame into an image (i.e., a screenful) but have only confused screen image for a while. Therefore, data of the first frame is not interframely but intraframely coded and stored. The recommendation H.261 proposes to periodically refresh a system component for preventing error accumulation due to interframe coding and to periodically conduct an intraframe coding of moving picture data. By using this opportunity, a sequence of the data frames are stored with an intraframely coded frame (data) placed at the head thereof. This makes it possible to reproduce video with a rapid feed by thinning the data units (frames) when reproducing the video data stored.
A video storage type communication device which realizes rapid-feed reproduction of images by using periodical refreshment is disclosed in Japanese Laid-Open Patent Publication No. 5-91497. The disclosed device is featured by that whenever a rapid feed control is carried-out, it can start from intraframely coded data, i.e., data of the heading (intraframely coded) frame of interframely coded frames, assuring the synchronized processing visual data and audio data.
The above-mentioned structure of stored data, however, is featured by coexistence of the data necessary for usual reproduction and the data necessary for rapid-feed reproduction. Therefore, it is required to control a storage and data for correctly distinguishing the above-mentioned two kinds of data from each other when reproducing them. Furthermore, the rapid-feed reproduction requires such reproduction control that may select and read only data necessary for rapid-feed reproduction. All these facts may require much complicated data control when storing and reproducing the data. In addition, as mentioned above, periodical refreshment depends upon terminals which have different period of intraframe coding, depriving the storage and communication device of the possibility of controlling a speed of rapid-feed reproduction.
Furthermore, the recommendation H.261 provides that video data coding is conducted on each of blocks into which one frame of video data is divided for encoding, whereas refreshment is conducted on a macroblock consisting of a plurality of the blocks which may not correspond to one frame. Therefore, some terminals may not guarantee that the whole of a frame is intraframely coded.
It is possible to force a terminal to intraframely encode a whole video frame and receive the intraframely coded frame therefrom by sending a signal requesting “fast update” from the video storage type communication device. However, since timing is not defined, the receiving side has to discriminate whether the received frames are intraframely coded or interframely coded by checking all frames one by one macroblock. Data control may be much complicated.